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　　Design considerations for electric vehicle charger circuit topology

　　With the development of modern high-tech and the increasingly prominent problems of environment and energy in today's world, electric-driven vehicles have become a hot spot in the research, development and use of the automobile industry. Since the 1980s, countries around the world have embarked on a large-scale development of electric vehicles. However, the marketization of electric vehicles has been troubled by some key technologies. Among them, one of the more prominent issues is the charging technology to ensure that electric vehicle battery packs are safe, efficient, user-friendly, sturdy, and cost-effective [1][2].

　　1Charging technology

　　Electric vehicle battery charging is one of the key technologies that must be solved before electric vehicles are put into the market. Electric vehicle battery charging generally uses two basic methods: contact charging and inductive coupling charging. 1.1 Contact charging

　　The contact charging method uses a traditional contactor, and the user connects the charging source connector to the car. A typical example is shown in Figure 1. The disadvantage of this method is that the conductor is exposed outside and is unsafe. Moreover, repeated plugging and unplugging operations will cause mechanical wear, resulting in loose contacts and inability to effectively transmit electrical energy. 1.2 Inductive coupling charging

　　Inductive coupling charging means that there is no direct electrical contact between the charging source and the vehicle receiving device, but a combination of separate high-frequency transformers, which transmit energy contactlessly through inductive coupling. The inductive coupling charging method can solve the shortcomings of the contact charging method [3][4].

　　Figure 2 shows a simplified power flow diagram of an electric vehicle inductively coupled charging system. In the figure, after the input grid alternating current is rectified, it passes through the high-frequency inverter link, is transmitted through the cable and passes through the inductive coupler, and then is transmitted to the input end of the electric vehicle, and then passes through the rectification and filtering link to charge the on-board battery of the electric vehicle.

　　The inductive coupling charging method can be further designed into a fully automatic charging method that does not require human intervention. That is, the primary and secondary sides of the magnetic coupling device of the inductive coupler are separated by a larger distance, and the charging source is installed at a fixed location. Once the car is parked in this fixed area, it can receive the energy from the charging source without contact. Realizes induction charging, thus achieving fully automatic charging without the intervention of car users or charging station staff. Figure 4 Principle block diagram of inductive coupling charging converter 2 Inductive coupling charging standard—SAEJ-1773

　　In order to realize the marketization of electric vehicles, the American Society of Automotive Engineers has formulated corresponding standards based on system requirements. Among them, two charging standards, SAEJ-1772 and SAEJ-1773, have been formulated for chargers of electric vehicles, corresponding to contact charging and inductive coupling charging respectively. Electric vehicle charging system manufacturers must comply with these standards when designing, developing and producing electric vehicle chargers.

　　The SAEJ-1773 standard provides the minimum physical size and electrical performance requirements for electric vehicle inductive charging couplers in the United States. Figure 5 Isolated Boost converter with two switching tubes The charging coupler consists of two parts: the coupler and the car socket. The combination is equivalent to a transformer with separated primary and secondary sides operating at a frequency between 80 and 300kHz.

　　For inductively coupled electric vehicle charging, SAEJ-1773 recommends three charging methods, as shown in Table 1. For different charging methods, the design of the charger will be different accordingly. Among them, the most commonly used method is home charging, with a charger power of 6.6kW. Chargers with higher power levels are generally used in charging stations and other occasions.

　　Table 1 Three charging modes recommended by SAEJ-1773 Charging modes

　　Charging mode Charging method Power level

　　Grid input

　　Mode 1 Mode 2 Mode 3 Emergency charging Home charging charging station charging 1.5kW6.6kW25～160kW

　　AC120V, 15A single-phase AC230V, 40A single-phase AC208~600V three-phase

　　According to the SAEJ-1773 standard, the inductive coupler can be represented by the equivalent circuit model shown in Figure 3. Corresponding component values are listed in Table 2.

　　Table 2 Component values of equivalent circuit model of inductive coupler for charging

　　fmin(100kHz)

　　fmax(350kHz)

　　Rpmax/mΩLp±10%/μHRsmax/kΩLs±10%/μHRmmin/mΩLm±10%/μHCs/μF turns ratio voltage per turn/V coupling efficiency/% insulation resistance/MΩ maximum charging current/A maximum charging voltage/V

　　200.81.645200.80.024:4100≮99.5100400474

　　400.51.355400.50.024:4100≮99.5100400474

　　The primary and secondary sides of the transformer are separated, have a large air gap, are loosely coupled magnetic parts, and the magnetizing inductance is relatively small. When designing the converter, the impact of this small magnetizing inductance on the circuit design must be fully considered [5].

　　Power transmission cables must still be considered in the design. Although this item is not included in the SAEJ-1773 standard, the volume, weight and equivalent circuit of the power transmission cable must be considered in the actual design. Since the size of the transmission cable is mainly related to the level of transmission current, reducing the charging current can correspondingly reduce the cable size. In order to minimize cable power loss, coaxial cables can be used and optimized in the operating frequency range. In addition, the cable will introduce additional impedance and increase the equivalent leakage inductance of the transformer. In the design of the power stage, its impact must be considered. For a 5m long coaxial cable, the typical resistance and inductance values are: Rcable=30mΩ; Lcable=0.5~1μH.

　　3 Requirements for inductively coupled charging converters

　　According to the equivalent circuit of the inductive coupler given in the SAEJ-1773 standard, the characteristics of the connecting cable and battery load, it can be concluded that the inductive coupling charging converter should meet the following design standards.

　　3.1 Current source high frequency chain

　　The secondary filter circuit of the inductively coupled charging converter is installed on the electric vehicle. Therefore, using a capacitive filter circuit in the filtering link will simplify the on-board circuit and thereby reduce the weight of the entire electric vehicle. For the capacitive filtering link, the converter should have high-frequency current source characteristics. In addition, this current source circuit is relatively less sensitive to changes in the operating frequency and power level of the converter. Therefore, it is easier to consider the three charging modes simultaneously for circuit design. Moreover, the secondary side uses a capacitive filter circuit, and the secondary side diode does not need to use overvoltage clamping measures. 3.2 Soft switching of main switching device

　　The higher frequency of the inductive coupling charging converter can reduce the size and weight of the inductive coupler and on-board filter components, thereby miniaturizing the power supply system. However, as the frequency continues to increase, the switching loss of converters using hard-switching operation will greatly increase, reducing the efficiency of the converter. Therefore, in order to achieve higher frequency and higher power level charging, it is necessary to ensure soft switching of the main switching device and reduce switching losses. Figure 8 and Figure 93.3 Constant frequency or narrow frequency range operation

　　The inductive coupling charging converter operates at a constant frequency or a narrow frequency range, which is conducive to the optimized design of magnetic components and filter capacitors. At the same time, it must avoid operating in the radio bandwidth and strictly control electromagnetic interference in this area. For variable frequency operation, light load corresponds to high-frequency operation, and heavy load corresponds to low-frequency operation, which is conducive to consistent efficiency under different load conditions.

　　3.4 Wide load range operation

　　Inductively coupled charging converters should be able to operate safely over a wide load range, including open-circuit and short-circuit extreme conditions. In addition, the converter should also be able to operate in trickle charging or equalizing charging modes. In these modes, the converter should be able to ensure high efficiency.

　　3.5 turns ratio of inductive coupler

　　A large primary-to-secondary turns ratio can make the primary-side current smaller, allowing the use of power transmission cables with thinner diameters and power devices with lower current ratings, thereby improving efficiency.

　　3.6 Input unit power factor

　　The inductively coupled charging converter works at high frequency and will cause harmonic pollution to the power grid. For inductive charging technology to be recognized by the public and widely used, effective measures must be taken, such as power factor correction or reactive power compensation, to limit the total harmonics entering the power grid from the inductive coupling charging converter of electric vehicles. For now, charging converters must meet the IEEE519?1992 standard or similar standards. To meet these standards, the input part of the inductive coupling charging converter and the whole machine are increased in complexity and cost. Moreover, according to different charging level requirements, the inductive coupling charging converter can choose a two-stage structure (the front stage is pFC + the rear stage is a charger circuit) or a single-stage circuit with integrated pFC function and charging function. 4Converter topology selection

　　Based on the equivalent circuit component values of the inductive coupler given by SAEJ-1773 and the above design considerations, the converter topology suitable for three different charging modes is examined here.

　　As shown in Figure 2, the on-board part of the electric vehicle includes the jack part of the inductive coupler and the AC/DC rectifier and capacitor filter circuit. First, the rectifier circuit directly connected to the capacitor filter is examined. Suitable rectification methods include half-wave rectification, center-tap full-wave rectification and full-bridge rectification. Among them, half-wave rectification has a low utilization rate of the transformer; full-wave rectification requires two windings connected with a center tap on the secondary side, which increases the weight and volume of the vehicle circuit; full-bridge rectification has a high utilization rate of the transformer and is more suitable for this application. kind of occasion.

　　Figure 4 shows the principle block diagram of the inductive coupling charging converter based on the above considerations. In the figure, the output rectification uses a full-bridge rectifier circuit, the output filter uses capacitor filtering, and a pFC circuit is used at the input end to limit the total harmonics entering the power grid from exceeding the standard. A separately designed pFC stage is used here. At low power, pFC can also be combined with the main charging converter to form an integrated charging circuit with pFC function. As mentioned before, a very important consideration in charger design is the reasonable selection of the turns ratio of the inductive coupler. To standardize the design, inductively coupled charging converters designed for the three charging modes must be able to use the same electric vehicle socket. Factors that limit the number of secondary turns of the charger's high-frequency transformer include wide power range, electrical design limitations, and mechanical design limitations. Typical coupler design is

　　The number of turns on the secondary side is 4. For low charge levels, a turns ratio of 1:1 is generally used, and for high charge levels, a turns ratio of 2:1 is generally used.

　　For energy storage capacity within 30kW·h, the battery voltage of electric vehicles changes in the range of DC200~450V depending on the charging state, and the converter topology should be able to provide the required charging current within this range of battery voltage changes.

　　4.1 Charging mode 1 This is an emergency charging mode for electric vehicles, charging slowly. Chargers designed in this mode are usually carried with electric vehicles and used when there is no standard charger, so they must be small in size, light in weight, and low in cost. According to these requirements, a single-stage high power factor converter can be used to reduce the volume, weight, and cost of the entire machine, and obtain higher overall efficiency. Figure 5 shows an alternative: an isolated Boost converter with two switching tubes [6]. When the auxiliary switch is not used, the single-stage Boost stage circuit provides the pFC function and regulates the output voltage. When the input voltage is AC120V, the peak value of the input voltage is 170V. Since the number of turns on the secondary side of the transformer is 4, the adjustment range of the output voltage is DC200~400V. Therefore, the transformer can use a turns ratio of 1:1, and the primary windings all use 4 turns. turns coil. Typical voltage and current waveforms are shown in Figure 6.

　　When the primary switches S1 and S2 are both turned on, energy is stored in the input filter inductor, and the output rectifier is in the off state. When either one of the switching tubes S1 and S2 is turned off, the stored energy is transmitted to the secondary side through the primary winding. Due to the symmetrical operation of the converter, the transformer flux is reset to balance.

　　In order to balance the input inductance volt-second product, (1) must be satisfied

　　Vinmax≤(Np/Ns)VB(1-Dmin)(1)

　　Assuming that the transformer turns ratio is 1:1 and the maximum input voltage is 170V, the duty cycle is 0.15 when the output voltage is DC200V, and the duty cycle is 0.5 when the output voltage is DC475V. As shown in Figure 5, the voltage stress on the main switch tube is 2VB. When the output voltage is DC400V, the voltage stress of the switch tube is DC800V, which is quite high. Also, device voltage stress may be higher due to leakage inductance of transmission cables and inductive couplers. In order to limit the maximum voltage stress of the device, the lossless absorption circuit shown in Figure 5 can be used. But in either case, devices with a voltage rating of 1200V must be used. Because the on-resistance of high-voltage MOSFET is high, the conduction loss will be large. Therefore, high-voltage IGBTs with low conduction voltage drop should be considered. However, the switching loss of IGBT devices also limits the increase in switching frequency.

　　The average current of the switch tube is

　　ISavg=(1/2)ILavg(2)

　　For the 1.5kW power level, the input current rms is 15A, the average switching current is 13A, and the peak current is 22A, requiring a switching device with a current rating of at least 30A. Although this solution provides a relatively simple single-stage power conversion, it also has some shortcomings, such as high voltage stress on semiconductor devices, poor output voltage regulation performance, and large output current ripple.

　　In order to reduce the switching loss of the device, the soft switching circuit shown in Figure 5 can be used. The turn-off delay designed for the MOSFET ensures the ZVS turn-off of the IGBT. In the current rising mode, the MOSFET shares the output filter current, and its voltage stress is half that of the IGBT. Therefore, 600V devices can be used. At the same time, the switching frequency can be increased due to the reduction of turn-off losses.

　　Another solution to reduce the voltage rating of the device is to use a two-stage conversion structure. The front-stage pFC correction link can use a Boost converter with soft switching function, allowing high-frequency operation. The subsequent DC/DC power conversion stage can use a half-bridge series resonant converter to provide a high-frequency current chain. Figure 7 shows the structure diagram of the two-stage power conversion circuit suitable for charging mode 1.

　　If the input grid voltage is AC115V, in order to reduce the current rating of the DC/DC converter, the output voltage can be increased to DC450V. In this way, Boost level power switch tube500~600V MOSFETs can be used, and the switching devices of the half-bridge converter can use 300~400V MOSFETs. Due to the half-bridge operation, the inductive coupler can use a turns ratio of 1:2. If the primary winding has 4 turns, the secondary winding has 8 turns. The current rating of the Boost switch tube is 30A, while the current rating of the half-bridge converter switch tube is 20A. 4.2 Charging mode 2

　　This is a normal charging mode for electric vehicles. The charging process is generally carried out at home and in public places, and it is required to provide users with a good user interface.

　　The charging power level of charging mode 2 is 6.6kW.

　　The standard grid power supply of 230V/30A specification is sufficient to power this load. Its typical charging time is 5 to 8 hours.

　　Similar to the charging power converter in charging mode 1, charging mode 2 can also use a single-stage AC/DC converter. However, due to the single-stage converter with pFC function, the peak current of the switch tube is very high, so it is best to use a two-stage converter. Among them, the pFC stage can use the traditional Boost circuit, and the switching tube can use either soft switching or hard switching. But in order to improve efficiency, soft-switching Boost converters are preferred. Figure 8 shows two main circuit power stages of soft open tube Boost converters using lossless absorption circuits. Figure 9 shows two soft open tube Boost converter power stages using active switching auxiliary circuits [7][8].

　　If the grid input voltage is 230V, the output voltage can be adjusted to more than 400V. This makes the design of the subsequent converter easy, and the inductive coupler can have a turns ratio of 1:1. Therefore, if the maximum battery voltage is 400V, the front-end output voltage can be DC450V.

　　Compared with the power stage of a soft open-tube Boost converter with an additional active switching auxiliary circuit, the power stage of a lossless absorbing soft-open tube Boost converter has more advantages because it does not require active components. Especially in Figure 8(b), the turn-off dv/dt of the switch tube is controlled, the turn-on is zero voltage turn-on, and the voltage stress on the main switch tube is the output voltage, so the performance of the whole machine is greatly improved. Figure 10 shows a typical waveform of a lossless absorption circuit. For a power rating of 6.6kW and an output voltage of 450V, a 600V/60A MOSFET is required. Depending on the needs of the application, single module or multi-module parallel solutions can be selected for the complete machine design.

　　For the downstream DC/DC converter, since the input and output are both capacitive filters, only high-frequency converters with current source characteristics are suitable. The following topologies with large inductors connected in series with the primary side of the transformer are suitable for use. One form is the full-bridge converter shown in Figure 11.

　　A series inductor is used in the primary circuit, so that the leakage inductance of the inductive coupler is effectively utilized, and the magnetizing inductor can also be used to expand the operating range of the converter ZVS. For an input bus voltage of 450V, a turns ratio of 1:1 can be used, that is, both the primary winding and the secondary winding use 4-turn coils.

　　One of the disadvantages of the bridge structure converter topology is the high peak current, especially at low voltage input. In addition, when corresponding to light load, the converter enters an intermittent working state, the turn-on loss of the main switch tube increases, and the regulation characteristics become worse. Therefore, it is usually necessary to ensure a minimum load current to ensure ZVS.

　　Another type of converter topology with high-frequency current source characteristics is the resonant converter. Literature [8] classifies these converter topologies into current type and voltage type. In a current mode converter, the converter is powered by a current source. In this type of topology, the current is effectively controlled. But its disadvantage is that the voltage on the switching tube is not effectively controlled. Because most power devices can withstand overcurrent better than overvoltage.

　　In addition, in voltage source converters, the voltage of the switching devices is well limited, but in full-bridge and half-bridge topologies, it may be damaged by breakdown. These converters are usually divided into three types: series, parallel and series-parallel resonance.

　　Figure 12 gives a schematic diagram of these basic resonant converter topologies. In a series resonant converter, the resonant inductor is connected in series with the primary side of the transformer, while in other types of converters, the capacitor is connected in series with the transformer. Only series resonant converters have hard current source characteristics, while other types of converters have hard voltage source characteristics.

　　To effectively utilize the inductive coupler magnetizing inductance and inter-turn capacitance, different series resonant converters can be used. One topology is the series-parallel LLCC resonant converter shown in Figure 13 [9][10]. Other resonant converters can also be considered. As mentioned before, the interturn capacitance, magnetizing inductance and leakage inductance are fully utilized. This solution is attractive because the converter and inductive coupler are well matched.

　　The converter can operate in the ZVS state above the resonant frequency, or in the ZCS state below the resonant frequency, as shown in Figure 14. The output voltage can be controlled by frequency conversion. However, in order to optimize the performance of the inductive coupler, it is generally designed such that high frequency corresponds to light load operation and low frequency corresponds to heavy load operation, so that the switching loss of the converter remains basically constant within the frequency range.

　　Due to the boost characteristics of the parallel resonant circuit, the maximum converter voltage gain is slightly greater than 1. For an input voltage of 450V and an output voltage of 400V, a turns ratio of 1:1 can be used. The output voltage control characteristics of this kind of converter are relatively poor when operating at light load, and some other control technologies need to be used. One option is to use an input boost stage to regulate the output voltage, and another is to use pWM or phase-shift control. Both control technologies are introduced in detail in relevant literature.

　　4.3 Charging mode 3

　　This is a fast charging mode, mainly for long-distance travel situations. The charger corresponds to high power characteristics (>100kW) and is mainly used in some fixed charging stations. For a power level of 100kW, charging time is approximately 15 minutes.

　　In order to improve the power factor and reduce the harmonics of the input grid, the input end of the converter generally needs to use an active rectifier circuit, as shown in Figure 15. Different control schemes can be used, including vector control, six-step wave control, digital control technology, etc. [11].

　　In order to further improve the conversion efficiency and allow high-frequency operation, a ZVT circuit as shown in Figure 16 can be used. The auxiliary circuit is used to realize the ZVT of the main switching device, and the main switch is still pWM controlled.

　　As mentioned before, high-power charging mode is usually only used at charging stations. Because the charging station may be equipped with multiple chargers, using a separate rectification stage for each charger will inevitably make the system bulky and cost greatly. To simplify system design, the entire charging station can be equipped with a dedicated pFC or harmonic compensation converter, so that the main charging circuit is connected to the same active input rectifier circuit, as shown in Figure 17.

　　The active filter rating is approximately 20% of the charging station's rated power rating. At the rectifier end, a DC side inductor is generally used to improve the power factor of the rectifier, and an active filtering scheme in series or parallel can be selected.

　　The active filter can use a traditional hard-switching pWM inverter circuit or a soft-switching inverter to operate at a higher switching frequency, increase the control bandwidth, and compensate for higher-order harmonics. The resonant DC link converter is more suitable for working in a wide medium power range inverter application. Figure 18 shows the active clamp resonant DC link inverter power circuit.

　　Different from the traditional pWM converter, the resonant DC link inverter adopts discrete pulse modulation (DPM, DiscretepulseModulation) control, with higher switching frequency and smaller filter size required. In addition, since dv/dt is controlled, the EMI generated is smaller.

　　Similar to charging mode 2, the charging converter can directly use a full-bridge or a full-bridge converter with resonance. However, due to the higher power level of charging mode 3, compared with the resonant full-bridge converter, the general full-bridge converter will inevitably correspond to a higher peak current. Therefore, the ZVS or ZCS resonant full-bridge topology should be considered to effectively reduce losses. As mentioned earlier, the series-parallel full-bridge resonant converter is an alternative topology that meets all design considerations for inductively coupled charging converters and fully utilizes the equivalent circuit elements of the inductive coupler. Depending on the performance differences of power devices, ZVS or ZCS solutions can be selected respectively.

　　For high power levels and high frequency applications, IGBTs with relatively small conduction losses and high frequency capabilities are more attractive. Since the frequency range of the optimal design of the inductive coupler is 70 to 300kHz, soft switching technology is needed to optimize the performance of the IGBT. The results in literature [10] show that in the case of ZVS, the IGBT turn-off loss is still large and the die temperature is high; ZCS can make the IGBT turn off in the ZCS case, reducing the turn-off loss and making the IGBT more efficient. Good for use at high switching frequencies.

　　To further reduce device current stress and reduce transmission cable size and weight, higher levels of bus voltage can be used. At this time, the inductive coupler can use a turns ratio of 2:1. Therefore, when the secondary side uses 4 turns, the primary side should use 8 turns. For a 400V battery voltage, the DC bus voltage must be at least DC800V, and an IGBT with a rating of 1200V/400A must be used.

　　5 Conclusion

　　This article discusses chargers for electric vehicle power batteries based on the provisions of inductive couplers in SAEJ?1773. According to the standards of inductive couplers and different charging modes, several design solutions for chargers matching the inductive couplers were determined, and circuit topologies suitable for different charging modes were selected. Finally, alternative converter topology solutions suitable for different charging levels are given.

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